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In Reply to: Re: How do you mix 6 channels into 2, without a mixer? [Mapleshade does it] posted by mg16 on March 10, 2002 at 13:25:32:
...Mostly correct. The circuit you describe is a virtual earth circuit, and is right up to replacing the resistors with potentiometrs, also it is important that the non-inverting (+) input is connected to ground. The output will in fact be the *inverse* sum of the inputs, since this is an inverting configuration.It's called a 'virtual earth' ("earth" is the British term for 'ground') because the junction of the seven resistors will always be ground, since the Op-amp will always trim it's output to make the two inputs -inverting (-) and non inverting (+) the same. Hence, +1V at any input gives -1V at the output, and the mid-point of the resistors will be the mid-point between the input & output voltages... (i.e. zero volts) thus keeping the potential at the (-) input the same as the (+) input, which is tied to ground.
Here's the problem with *replacing* the resistors with potentiometers: -if you take any one (or more) of the wipers to ground, then the (-) input is held at ground, and the op-amp goes into open-loop gain. However, since this also shorts the combined summing point to ground, then you have no signal, but lots of noise!
The correct way to do it is to leave the summing resistors in place, and put the potential dividers *before* the summing resistors.
An additional benefit of the virtual ground circuit is that the input impedance of each input is precisely controlled (It's the same as the summing resistor at all times, since the othe end of the resistor is connected to a virtual ground) and also that there can be no 'back-bleed' between outputs, since the only way that they are coupled is through a virtual ground, which kills any crosstalk.
For microphone use, this is no good as a front end, since the virtual earth circuit is unbalanced, so a good balanced preamp must be used. This will also allow the signal level to be raised away from the circuit's internal noise.
A virtual Earth circuit's noise level is linked to it's gain, and it's gain is proportional to the number of inputs connected. -For this reason, it is advantageous to add switches to one end of the summing resistors. (Hence all Rupert Neve-designed consoles have buss de-assign switches to the main busses.)
Keith A.
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Follow Ups
- Re: How do you mix 6 channels into 2, without a mixer? [Mapleshade does it] - SSL tech 07:15:41 03/11/02 (0)
- Re: How do you mix 6 channels into 2, without a mixer? [Mapleshade does it] - Roland 12:00:52 03/11/02 (0)
- Re: How do you mix 6 channels into 2, without a mixer? [Mapleshade does it] - SSL Tech 07:12:47 03/12/02 (1)
- Re: How do you mix 6 channels into 2, without a mixer? [Mapleshade does it] - Roland 15:25:24 03/12/02 (0)
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